Heusler alloys have been known as a half-metallic ferromagnet. Heusler alloys include a full-Heusler alloy and a half-Heusler alloy. FIG. 1 is a diagram showing the L21-type crystal structure of CO2FeSi, which is a full-Heusler alloy. Thus, a full-Heusler alloy has the composition of X2YZ. A half-Heusler alloy has the C1b-type crystal structure and has the composition of XYZ. Here, elements such as Co (cobalt) and Ni (nickel) can be used as X. Elements such as Fe (iron), Cr (chromium), and Mn (manganese) can be used as Y. Elements such as Si (silicon) and Ge (germanium) can be used as Z. For example, the Curie temperature of CO2FeSi, which is a full-Heusler alloy, is not less than room temperature. Thus, CO2FeSi can be used as a half-metallic ferromagnet at room temperature. Heretofore, a Heusler alloy has been produced by a sputtering method or a molecular beam epitaxy method.
For example, a half-metallic ferromagnet is used as a source electrode and a drain electrode of a spin-transistor as disclosed in Non-Patent Literatures 1 and 2. Because the half-metallic ferromagnet can have a spin polarization of 100% at the Fermi level, a spin-transistor using a half-metallic ferromagnet for a source electrode and a drain electrode can obtain a high magnetocurrent ratio. On the other hand, Non-Patent Literatures 1 and 2 disclose a spin-transistor using an SOI (Silicon-on-Insulator) substrate.    [Non-Patent Literature 1] S. Sugahara, IEE Proc.—Circuits Devices Syst., Vol. 152, No. 4, 2005, pp. 355-365    [Non-Patent Literature 2] S. Sugahara, Phys. Stat. Sol. (c) 3, No. 12, 2006, pp. 4405-4413